fix(control-ui): create sessions for typed /new

Route typed Control UI `/new` through the dashboard session create-and-switch flow used by the New Chat button.

Keep typed `/reset` as the explicit in-place gateway reset path, and document the Control UI slash-command boundary.

Fixes #69599.

docs/tools/slash-commands.md

@@ -125,6 +125,7 @@ Current source-of-truth:
 <AccordionGroup>
   <Accordion title="Sessions and runs">
     - `/new [model]` starts a new session; `/reset` is the reset alias.
+    - Control UI intercepts typed `/new` to create and switch to a fresh dashboard session; typed `/reset` still runs the Gateway's in-place reset.
     - `/reset soft [message]` keeps the current transcript, drops reused CLI backend session ids, and reruns startup/system-prompt loading in-place.
     - `/compact [instructions]` compacts the session context. See [Compaction](/concepts/compaction).
     - `/stop` aborts the current run.

docs/web/control-ui.md

@@ -157,6 +157,7 @@ Imported themes are stored only in the current browser profile. They are not wri
     - During an active send and the final history refresh, the chat view keeps local optimistic user/assistant messages visible if `chat.history` briefly returns an older snapshot; the canonical transcript replaces those local messages once the Gateway history catches up.
     - `chat.inject` appends an assistant note to the session transcript and broadcasts a `chat` event for UI-only updates (no agent run, no channel delivery).
     - The chat header model and thinking pickers patch the active session immediately through `sessions.patch`; they are persistent session overrides, not one-turn-only send options.
+    - Typing `/new` in the Control UI creates and switches to the same fresh dashboard session as New Chat. Typing `/reset` keeps the Gateway's explicit in-place reset for the current session.
     - The chat model picker requests the Gateway's configured model view. If `agents.defaults.models` is present, that allowlist drives the picker. Otherwise the picker shows explicit `models.providers.*.models` entries plus providers with usable auth. The full catalog stays available through the debug `models.list` RPC with `view: "all"`.
     - When fresh Gateway session usage reports show high context pressure, the chat composer area shows a context notice and, at recommended compaction levels, a compact button that runs the normal session compaction path. Stale token snapshots are hidden until the Gateway reports fresh usage again.